Heart disease is one of the major causes of death in the world today. Health care costs associated with cardiovascular disease exceed those of any other disease. Sudden cardiac death (unexpected death due to a cardiac malfunction within a short time period from the onset of symptoms in an individual who would otherwise be considered to be a normal, healthy adult) is often the first manifestation of heart disease.
More than one in five heart attacks in people over the age of 65 are unrecognised, according to a study published in the January 2000 issue of the Journal of the American College of Cardiology. Of the 901 subjects in whom an electrocardiogram indicated a prior heart attack, more than one fifth had had heart attacks that had gone undetected until the test was conducted. Most patients had no clear indication of cardiovascular disease when they commenced the study. Lack of recognition may be based upon the true absence of symptoms (probably rare), unusual symptoms (for example, shortness of breath without chest pain) or misinterpretation of symptoms (for example, attribution of chest pain to “indigestion”).
Silent heart attacks are an extreme case of a condition called “silent ischemia”, which is a chronic shortage of oxygen and nutrients to a portion of the heart. Occurrences of silent ischemia increase the risk of sudden cardiac death.
The cause of ischemia, silent or otherwise, is almost always atherosclerosis—the progressive narrowing of the arteries to the heart from accumulations of cholesterol plaque. In most instances, this reduction in blood supply generates a “protest” from the heart—typically a crushing sensation called angina. However, in up to 30 percent of heart attack victims there are no previous symptoms associated with such blockages. This underscores the importance of early detection of coronary artery disease. Indeed, the Framingham heart study, which has followed 4,000 Massachusetts men for more than 40 years, has found that 25 percent of their subjects' heart attacks go unnoticed until their annual electrocardiograms detected the after-effects of those attacks.
Because these silent heart attacks go undetected, they cannot be treated early. This increases the chance of further—sometimes fatal—complications. According to the National Heart Foundation report (1999), 40% of sudden cardiac deaths are unpredictable, suggesting that many people are effectively carrying a time bomb. Almost 50% of heart attacks occur among people who have normal cholesterol, low blood pressure and are in good physical shape. For many people sudden death is the first manifestation of heart disease. Therefore early awareness and recognition are important to prevent sudden cardiac death.
There is no way of predicting with certainty who is a candidate for silent ischemia, but statistically, the greater the number of risk factors a patient possesses the more likely that he or she is a candidate. Not all cases of sudden cardiac death result from “heart attacks”. For example, patients with poorly functioning hearts are at risk of dangerous rhythm abnormalities. These life-threatening events may occur with few preceding symptoms. To be able to detect heart disease at an early stage is of great importance.
The basic tool for evaluation of the health of the human heart is the standard 12-lead electrocardiogram (ECG), which records the electrical activity of the heart, as detected by electrodes, located at established locations on the body surface, to which the leads are attached. The ECG is a commonly used cardiac diagnostic tool. Medical practitioners and cardiologists alike use the ECG to observe the electrical activity of the heart for determining whether any indications as to heart disease exist. It is thus widely accepted that the ECG is an extremely useful and, importantly, non-invasive diagnostic tool. Indeed, it is the only practical non-invasive method of recording the electrical activity of the heart, and importantly, it is the first laboratory test performed in a patient with chest pain, syncope or presyncope, the two major markers of potential cardiovascular catastrophe.
The ECG reflects an electrical phenomenon that can be altered identically by a variety of functional and anatomic disorders. It is recognised, however, that the resting ECG is not a very sensitive or specific marker for occult heart disease, and accordingly the prognostic value of many ECG abnormalities is unclear. Limitations of the use of the ECG for diagnosis include the difficulty of recognising small changes in voltage and time intervals, and the dependence upon deductive analysis by a specialist. Furthermore, intracardiac studies demonstrate that changes of conduction or cycle length in the order of few milliseconds may result in delay or blockage of an impulse, and such changes may not be evident from a surface ECG.
The available methods of ECG analysis are presently limited, and current diagnostic procedures are largely based on visual interpretation of the ECG signal by a skilled medical practitioner. Such interpretation of the ECG is a process requiring detailed visual analysis of the frequency and amplitude of signal elements and signal patterns representing the heart muscle function in operation. Furthermore, reading an individual ECG only provides an indication of heart function at a particular moment in time, and is highly dependent upon the expertise and experience of the medical practitioner viewing a print of the patient's ECG signal. The process of manually analysing an ECG signal may be time consuming, and is to a large extent subjective.
Presently, the most common form of assessment is based on an analysis of the resting ECG. However, it is known that patients with clinically significant coronary artery disease often have a normal resting ECG. Further, clinical studies have shown that 25% to 50% of patients with a history of chest pain due to documented coronary artery disease have normal resting ECG when they are pain free.
Known methods of heart assessment are generally based on the visual analysis of the resting ECG in the time domain. The technique involves consideration of the shape of the various complexes and segments making up the ECG waveform, and the counting of features such as notches and/or slurs in an ECG trace. The technique may also include investigation of such parameters as conduction intervals of electrocardial signals through the tissue. Such features and parameters may only become visible to the experienced medical practitioner or cardiologist once a problem is already present that signifies irreversible pathology.
An example human ECG is shown in FIG. 1. As can be seen, the heart rhythms traced by the ECG are rhythmic and repeating. The variation in amplitude of the electrical signal is due to the polarisation and depolarisation of different areas of the heart as part of each rhythmic pumping cycle. While the ECG generated depends on numerous factors, a skilled cardiologist will nonetheless often be able to notice anomalies in an otherwise normal looking ECG. However, reading an individual ECG effectively provides only a “static” picture of the heart function over a particular short time period, such as a few minutes, and does not allow for analysis of historical trends in a patient's heart function.
The ECG trace shown in FIG. 1 details several peaks and troughs in the amplitude of the electrical signals associated with the normal supraventricular rhythm and these are labelled sequentially with the letters P, Q, R, S, T and U within each rhythmic period. Analysis of the amplitude characteristics of particular parts of the rhythm can provide information concerning heart function for particular parts of the heart pumping cycle. The portion of the cycle which starts at the beginning of the trough marked Q and ends after the trough marked S is termed the “QRS complex” or “QRS interval”. It is known that the prolongation or shortening of the QRS complex relates to changes in the ventricular conduction velocity within the His-Purkinje system and indicates pathological processes associated with ischemia or infarction. The accuracy of QRS interval determination presents a problem concerning time domain analysis of ventricular rhythms. Visual estimation of the duration of the QRS complex can be relatively accurate for approximation purposes, but precise localisation of onset and offset of the QRS complex to within a few milliseconds can be difficult, particularly at more rapid heart rates and where signal noise is present. QRS interval measurements derived from visual estimation may also be affected by different abilities among medical practitioners and consequent limitations in subjective accuracy.
The portion of the cycle which starts at the beginning of the trough marked S and ends at the peak marked T is termed the “ST segment”. Traditionally, changes to the ST segment and the T wave have been utilized for detection of myocardial ischemia or infarction. In considering the ST segment, most practitioners employ one of two standards for determining the portion of the ECG waveform for closest analysis. The first of these standards involves consideration of the period of 60 milliseconds following the point S of the ECG waveform, whereas the alternative standard involves consideration of the period of 80 milliseconds following this point.
Of course, other complexes and/or segments of the ECG waveform also include information relevant to the analysis of heart function.
In view of the aforementioned limitations of conventional time domain analysis, spectral evaluation of the ECG signal in the frequency domain has been proposed. However, no method of spectral evaluation as yet been widely accepted as an alternative to the conventional analysis.
Further methods of ECG analysis that have been studied include those directed to variability in heart rate, detection of late potentials, changes in R wave amplitudes, and various spectral mapping techniques. As yet, none of these have been found to be as useful or effective for assessment of heart function as current methods, and their clinical use is as yet unproven.
Accordingly, known methods of heart function analysis while useful, suffer from a number of limitations. These include the fact that these methods are inherently static, in that they consider an individual ECG that can only provide a picture of heart function at a particular moment in time. Furthermore, the assessment depends upon the expertise of specialists performing a visual analysis of ECG traces, which may be subjective, and of limited sensitivity. Furthermore, in order to facilitate such visual assessment, standard equipment incorporates filters and/or amplifiers, which may exclude potentially significant information and/or obscure signals of interest. It may be difficult to discriminate between noise and signs of possible disfunction such as late potentials. Additionally, inconsistent definitions of abnormal findings may be applied by different medical practitioners.
However, despite these limitations, improved methods of non-invasive heart monitoring have yet to be widely introduced.
In light of the above, it is desired to provide a method and system which enables objective analysis of ECG data relating to complexes and/or segments of the ECG signal by providing a numerical indication concerning heart function. Such an indication would assist medical practitioners to determine whether a patient may be at risk of being affected by heart disease.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the claims herein.